A conventional reciprocating internal combustion engine facilitates combustion within its cylinders by using valves to control air and fuel flow into and out of the cylinders. To control the opening and closing of the valves, engines use a “valve train”, which is an assembly of components that transmit motion throughout the assembly.
Depending on its type, a valve train may consist of various valves, rocker arms, pushrods, tappet lifters, and camshaft(s). The opening, closing and duration of valves control the amount of air and fuel entering the combustion chamber at any given point in time. Timing for valve opening, closing and duration is controlled by the camshaft, which is synchronized to the crankshaft by a chain, belt or gear.
Tappet lifters (also called simply tappets or lifters or cam followers) are made to move vertically by the action of the cam. In overhead valve (as opposed to overhead cam) valve trains, each tappet lifter drives a long thin pushrod, up to the top of the engine above the cylinder heads. The camshaft pushes on the tappet lifter, which pushes on the pushrod, which pushes on a rocker arm, which pivots and pushes down on the valve. In contrast, in overhead cam valve trains, some are “direct-acting” so that there may or may not be rocker arms and pushrods.